Recent advances in adjuvant therapy for malignant bone and soft tissue tumors, the use of modern imaging techniques, and a surgical staging system have greatly improved the outlook of limb sparing procedures. The use of a modular prosthetic system based on established design principles and material, and the adoption of a new composite cement/porous ingrowth fixation concept to reconstruct skeletal defects have shown great promise. Initial implant fixation is achieved using bone cement. Long-term fixation will rely upon extracortical bone bridging and ingrowth over the porous-coated shoulder section of the prosthesis. The specific aims of this investigation are: 1) to optimize the modular system design for versatile application, safe mechanical strength, and strong intramedullary cement fixation; 2) to seek autogenous bone graft substitutes to enhance extracortical bone bridging and formation; 3) to develop a satistfactory soft tissue attachment method to the segmental bone/joint prosthesis; and 4) to follow patients with such implants and compare their results with other limb salvage procedures. Size optimization of the modular implant system will be achieved based on our past clinical experience and the normal long bone dimensional data accumulated. Implant and fixation strength will be tested in vitro under bending and torsional loading conditions. Finite element analysis will be adopted to study the stress transfer phenomenon with and without extracortical bone bridging. We will study the efficacy of alternative bone grafting materials to stimulate extracortical bone formation. A custom total knee prosthesis with a porous-coated pad on the anterior surface of the tibial component will be used to study the patellar tendon reattachment problem. Clinical weight-bearing, radiographic, histomorphologic and biomechanical techniques will be used to study the results. A multi-institutional clinical trial and follow-up on the developed modular prosthetic systems will be conducted using qualitative functional and device evaluation systems recommended by the Musculoskeletal Tumor Society. Objective functional evaluation and radiographic analysis will also be performed for all limb- salvage patients on a yearly basis so that the results of different reconstructive techniques and devices can be compared. A successful development of this implant system and the fixation concept will have significant application in general reconstructive orthopedic problems.